306 M. Becquerel on the Electric effects of Contact 



temperature till the instant when the combination takes place ; 

 that fire then bursts out, produced by the simultaneous com- 

 bination of the two electricities, and that all the electric phe- 

 nomena soon cease. 



In this theory only one point has been demonstrated by ex- 

 periment, and that is the electric condition of acid and alkaline 

 bodies on their mutual contact, but we are quite ignorant of what 

 takes place when their temperature is simultaneously varied. 



I shall now describe the process by which I have measured 

 the electro-dynamic force produced by an electric current, 

 which describes a metallic circuit enveloped with a silk thread, 

 and rolled round a box so as to form a galvanometer, in which 

 is placed a system of two magnetised needles, as devised by 

 M. Ampere. A divided circle upon a plate of glass points 

 out the deviations of one of the needles. The first point is to 

 ascertain the ratios between- these deviations, and the corre- 

 sponding intensities of the electro-dynamic force. 



This intensity has been- supposed proportional to the size of 

 the angle of deviation, but this law is not founded on any ex- 

 periment. My object was to determine the intensity of the 

 current which corresponds to a given deviation. 



By employing two magnetised needles fixed in a parallel po- 

 sition, and with their opposite poles near one another, one in 

 the inside of the box, and the other without it, we destroy, in 

 a great measure, the influence of terrestrial magnetism, and 

 we leave them no other directive force but what is necessary 

 to bring them back into their ordinary position of equilibrium, 

 when they are made to deviate from it. Its sensibility is such, 

 that when we employ a divided brass circle, crossed by a bar 

 of the same metal, the magnetic needle, when made to oscil- 

 late, will place itself in the direction of this bar. This source 

 of error, therefore, is avoided by making use of a circle di- 

 vided upon glass. 



Instead, however, of one wire of copper, I take three of 

 the same metal, equal in length and in diameter, equally co- 

 vered with silk, and rolled in the same manner round the ap- 

 paratus. If we cause the same quantity of electricity to pass 

 into each of these wires, it is perfectly evident, that every thing 

 being similar on all sides we shall have three perfectly equal 



